The present invention is directed to a new process for the synthesis of benzo[b]thiophenes, in particular 2-arylbenzo [b]thiophenes.
Benzo[b]thiophenes have been prepared by a number of different synthetic routes. One of the most widely used methods is the oxidative cyclization of o-mercaptocinnamic acids. This route is limited to the preparation of benzo[b]-thiophene-2-carboxylates. 2-Phenylbenzo[b]thiophenes are prepared by acid-catalyzed cyclization of 2-phenylthioacetaldehyde dialkyl acetals. Unsubstituted benzo[b]thiophenes are prepared by catalytic condensation of styrene and sulfur. 3-Substituted benzo[b]thiophenes are prepared by acid-catalyzed cyclization of arylthiomethyl ketones; however, this route is limited to the preparation of 3-alkylbenzo[b]thiophenes. See Campaigne, "Thiophenes and their Benzo Derivatives: (iii) Synthesis and Applications," in Comprehensive Heterocyclic Chemistry (Katritzky and Rees, eds.), Volume IV, Part III, 863-934 (1984). 3-Chloro-2-phenylbenzo[b]thiophene is prepared by the reaction of diphenylacetylene with sulfur dichloride. Barton and Zika, J. Org. Chem., 35, 1729-1733 (1970). Benzo[b]thiophenes have also been prepared by pyrolysis of styryl sulfoxides. However, low yields and extremely high temperatures make this route unsuitable for production-scale syntheses. See Ando, J. Chem. Soc., Chem. Comm., 704-705 (1975).
The preparation of 6-hydroxy-2-(4-hydroxyphenyl)benzo-[b]thiophenes was described in U.S. Pat. Nos. 4,133,814 and 4,380,635. One process described in these patents is the acid-catalyzed intramolecular cyclization/rearrangement of .alpha.-(3-methoxyphenylthio)-4-methoxyacetophenone. The reaction of this starting compound in neat polyphosphoric acid at about 85.degree. C. to about 90.degree. C. gives an approximate 3:1 mixture of two regioisomeric products: 6-methoxy-2-(4-methoxyphenyl)-benzo[b]thiophene and 4-methoxy-2-(4-methoxyphenyl)benzo[b]-thiophene. These isomeric benzo[b]thiophenes co-precipitate from the reaction mixture, producing a mixture containing both compounds. To obtain a single regioisomer, the regioisomers must be separated, such as by chromatography or fractional crystallization. Therefore, there currently exists a need for an efficient and regiospecific synthesis of 2-arylbenzo[b]thiophenes from readily available starting materials.